It is very easy to accidentally trigger tiling by moving the window near the upper edge of the screen, especially if there is no vertical panel. The user attempts to snap the window to the upper edge of the screen but the window tiles instead. This is a common operation and there are only several pixels between the pointer and the tiling threshold. A similar issue occurs during tiling the window to other edges of the screen, e.g. if the user drags the window with an Alt key with the pointer near the window edge. This scenario is much much less likely to occur so solving only the upper edge problem might be sufficient.
Fixed in git master, closing.